Metal-free organic dyes that luminesce in the solid state are not common but have important applications in fields such as display technologies, sensing, nano-patterning, and solid-state lasers. Lately, new compounds such as dicyanopyrazines, 5-aryl-2,2′-bipyridyls, diazepines, heterocyclic quinol fluorophores, 2-aryl-3-hydroxyquinolones, fumaronitriles, dithienopyrroles, oxadiazoles, diborylphenylenes, pyrones, and perylenediimides have been reported to show solid-state fluorescence. However, many of these systems are synthetically complicated and employ materials that have limited industrial potential.
See for example:    Forrest, S. R.; Thompson, M. E. Chem. Rev. 2007, 107, 923-925.    Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864-11873.    Menard, E.; Meitl, M. A.; Sun, Y.; Park, J.-U.; Shir, D. J.-L.; Nam, Y.-S.; Jeon,    S.; Rogers, J. A. Chem. Rev. 2007, 107, 1117-1160.    Samuel, I. D. W.; Turnbull, G. A. Chem. Rev. 2007, 107, 1272-1295.    Park, S.-Y.; Ebihara, M.; Kubota, Y.; Funabiki, K.; Matsui, M. Dyes Pigm. 2009, 82, 258-267.    Karlsson, I.; Hillerström, L.; Stenfeldt, A.-L.; M{dot over (a)}rtensson, J.; Börje, A. Chem. Res. in Toxicol 2009, 22, 1881-1892.    Chatelain, E.; Gabard, B. Photochem. Photobiol. 2001, 74, 401-406.    Barry, J.; Fritz, M.; Brender, J. R.; Smith, P. E. S.; Lee, D.-K.; Ramamoorthy, A. J. Am. Chem. Soc. 2009, 131, 4490-4498.    Ran, C.; Xu, X.; Raymond, S. B.; Ferrara, B. J.; Neal, K.; Bacskai, B. J.;    Medarova, Z.; Moore, A. J. Am. Chem. Soc. 2009, 131, 15257-15261.
The solid-state fluorescence of organic molecules strongly depends on the molecular structure and intermolecular interactions present in different morphologies. Various emission colors may be achieved from the same fluorophore by taking advantage of material polymorphism, which is controllable by processing methods. See, for example:    Schwoerer, M.; Wolf, H. C. Organic Molecular Solids, WILEY-VCH, Weinheim, 2007;    Mutai, T.; Satou, H.; Araki, K. Nat. Mater. 2005, 4, 685-7;    Zhang, H.; Zhang, Z.; Ye, K.; Zhang, J.; Wang, Y. Adv. Mater. 2006, 18, 2369-72;    Mizukami, S.; Houjou, H.; Sugaya, K.; Koyama, E.; Tokuhisa, H.; Sasaki, T.; Kanesato, M. Chem. Mater. 2005, 17, 50-6;    Kohmoto, S.; Tsuyuki, R.; Masu, H.; Azumaya, I.; Kishikawa, K. Tetrahedron Lett. 2008, 49, 39-43.
Another more unusual strategy involves force-induced emission color changes, or mechanochromic luminescence. Ito et al. (Ito, H.; Saito, T.; Oshima, N.; Kitamura, N.; Ishizaka, S.; Hinatsu, Y.; Wakeshima, M.; Kato, M.; Tsuge, K.; Sawamura, M. J. Am. Chem. Soc. 2008, 130, 10044-5) reported that the photoluminescence of [(C6F5Au)2(μ-1,4-diisocyanobenzene)] can be switched from blue to yellow by grinding the solid. Ordered aggregates and an amorphous solid are believed to be responsible for the blue-shifted and red-shifted emission colors, respectively. Mechanochromic luminescence has also been attributed to the disruption of hydrogen bonding, resulting in a less ordered polymorph (Sagara, Y.; Mutai, T.; Yoshikawa, I.; Araki, K. J. Am. Chem. Soc. 2007, 129, 1520-1). More recently, Chung et al. (Chung, J. W.; You, Y.; Huh, H. S.; An, B.-K.; Yoon, S.-J.; Kim, S. H.; Lee, S. W.; Park, S. Y. J. Am. Chem. Soc. 2009, 131, 8163-72) reported fluorescence “turn on” by either UV irradiation or shear force for a cyanostilbene system.
Mechanochromic luminescence is rare, and to our knowledge systems such as we disclose and claim below, that are reversible at room temperature, are unprecedented. For previously described systems, redissolving the ground solid and drying, heating, exposure to solvent vapor or other methods were required to erase the marks and revert to the initial, ordered state. For examples see: Sagara, Y.; Kato, T. “Mechanically Inducted Luminescence Changes in Molecular Assemblies” Nat. Chem. 2009, 1, 605-10.